Mandrels are known from U.S. Pat. No. 4,381,709, issued May 3, 1983, which incorporate hydraulically actuable expansion heads, whereby a printing cylinder can be passed over the expansion heads and then locked in position on the mandrel by actuating one or more hydraulic actuators to cause the expansion heads to expand radially outwardly into locking frictional engagement with the interior surface of the printing cylinder.
The printing cylinders are of larger internal diameter than the external diameter of the mandrel itself, and only slightly larger than the external diameter of the expandable heads. Thus, a dead air space is provided between the mandrel proper and the printing cylinder.
The printing cylinder is employed to support printing plates which are secured directly to the outer surface of the printing cylinder. Thus, a costly printing cylinder must be employed in each printing run, the only option being or removing that cylinder and the attached printing plates and substituting another equally costly cylinder having different printing plates attached thereto.
The printing cylinders themselves are costly in that accurate machining thereof is required in order to ensure axial linearity and radial concentrically of the printing cylinders relative to their supporting mandrel.
The consequence of this known technology is that a very considerable number of costly printing cylinders must be kept in inventory, with the consequent expense of warehousing those cylinders.
More recent developments in the printing industry have provided a partial solution to the problem by providing a tubular cylindrical sheath of a resilient plastics material which is a force fit over the printing cylinder, and to which the required assemblage of printing plates is attached.
The sheath is removable from the printing cylinder, or in the alternative positionable thereon, by expanding the sheath radially outwardly under the action of compressed air fed into the printing cylinder and which escapes through radial perforations in the axial wall of the printing cylinder. In this manner, a cushion of pressurized air is provided within the sheath which acts to expand the sheath radially outwardly to break its frictional contact with the outer surface of the printing cylinder, this in turn permitting the sheath to be slid onto the printing cylinder or removed therefrom in an axial direction.
While this technique still requires warehousing of the sheaths themselves and the attached printing plates, the cost of providing a corresponding number of costly printing cylinders is eliminated in that a single printing cylinder can be employed in combination with any one of a number of relatively inexpensive sheaths.
While the savings are substantial, the technique carries with it the need to provide a relatively costly and cumbersome compressed air jig that provides for the closure of the respective ends of the printing cylinder, and in turn permits pressurization of the interior thereof.
Further, this technique carries with it the disadvantage that the printing cylinder itself must be removed from the mandrel in order for it to be inserted in the compressed air jig, and, subsequent to replacement of the sheath, it must then be repositioned and locked onto the mandrel, this increasing down time and manual labor in the removal and exchange of the sheaths.